We wish to understand the regulated expression of the globin and immunoglobulin genes of man and the mouse. In order to do this we have cloned the human and murine kappa and lambda light chain and heavy chain genes; the alpha and beta globin genes and pseudogenes of the mouse were also cloned. Detailed structural studies have been carried out that allow us to make certain critical inferences about mechanisms and nucleotide sequences that are involved in the transcriptional activation, somatic rearrangement and evolution of some of these genetic sequences. In particular we have established the arrangement of human immunoglobulin light and heavy chain locus that accounts for the extraordinary diversity that can be generated by this system; its major source arises from the alternative assembly of genes from subgenic fragments of DNA. In addition, using the globin gene system, we have established an evolutionary mechanism that depends upon the amplification and conveyance of gene sequences on and between the different chromosomes.